Lithium secondary batteries have characteristics that the energy density is high, its lifespan is long, and the like. Hence, lithium secondary batteries are widely used as a power source for home appliances such as a video camera, portable electronic devices such as a notebook computer and a mobile phone, and electric tools such as power tools, and they have also been recently applied to a large-sized battery that is mounted in an electric vehicle (EV) or a hybrid electric vehicle (HEV).
Lithium secondary batteries are a secondary battery having a structure in which lithium dissolves out from the positive electrode as an ion, moves to the negative electrode, and is absorbed therein at the time of charge and the lithium ion conversely returns from the negative electrode to the positive electrode at the time of discharge, and the high energy density thereof is known to be due to the potential of the positive electrode material.
As the positive electrode active material of the lithium secondary battery of this kind, a spinel-type lithium manganese-containing composite oxide having a manganese-based spinel structure (Fd-3m) such as LiMn2O4 and LiNi0.5Mn1.5O4 is known in addition to a lithium transition metal oxide having a layer structure such as LiCoO2, LiNiO2, and LiMnO2.
The spinel-type lithium manganese-containing composite oxide has attracted attention as the next generation positive electrode active material for a large-sized battery of an electric vehicle (EV), a hybrid electric vehicle (HEV), or the like since it is non-toxic and safe, has nature to be strong to overcharge, and the raw material price of which is inexpensive. In addition, the spinel-type lithium transition metal oxide (LMO) capable of three-dimensionally inserting and releasing a Li ion exhibits excellent output characteristics compared to a lithium transition metal oxide having a layer structure such as LiCoO2, and thus it is expected to be used in an application requiring excellent output characteristics such as an EV battery and a HEV battery.
In recent years, it has been known to have an operating potential of about 5 V by substituting a part of the Mn sites in LiMn2O4 with other metals (Cr, Co, Ni, Fe, Cu, and the like), and at present, the development of manganese-based spinel-type lithium transition metal oxide having an operating potential of 4.5 V or more (5 V-class) is being actively carried out.
For example, a spinel-type lithium manganese composite oxide positive electrode active material having a high capacity obtained by adding chromium as the essential additive component and further adding nickel or cobalt to a spinel-type lithium manganese composite oxide is disclosed as a positive electrode active material of a lithium secondary battery having an electromotive force of a 5 V-class in Patent Document 1.
A crystal LiMn2−y−zNiyMzO4 (where, M: at least one kind selected from the group consisting of Fe, Co, Ti, V, Mg, Zn, Ga, Nb, Mo, and Cu, 0.25≤y≤0.6, and 0≤z≤0.1) of a spinel structure which conducts charge and discharge at a potential of 4.5 V or more with respect to the Li metal is disclosed in Patent Document 2.
A spinel-type lithium manganese composite oxide represented by Lia(MxMn2−x−yAy)O4 (in Formula, 0.4<x, 0<y, x+y<2, and 0<a<1.2. M includes one or more kinds of metal elements which are selected from the group consisting of Ni, Co, Fe, Cr, and Cu and at least include Ni. A includes at least one kind of metal element selected from Si or Ti; however, the value of the ratio y of A is 0.1<y in a case in which A includes only Ti) is disclosed as a positive electrode material for lithium ion secondary battery having a high energy density so as to have a high voltage of 4.5 V or more with respect to Li in Patent Document 3.
A lithium nickel manganese composite oxide which has a spinel structure and is represented by Formula (I): Li1+xNi0.5−1/4x−1/4yMn1.5−3/4x−3/4yByO4 (where, in Formula (I), x and y are to be 0≤x≤0.025 and 0<y≤0.01) and characterized in that a median diameter is from 5 to 20 μm, a coefficient of particle size variation is from 2.0 to 3.5%, and a BET specific surface area is from 0.30 to 1.30 m/g is disclosed as a positive electrode active material having a high capacity density as both of the tap density of the positive electrode active material and the initial discharge capacity of a secondary battery fabricated using the positive electrode active material are high in Patent Document 4.
However, a problem that the electrolytic solution is decomposed and generate a gas in some cases has been pointed out in the case of using a lithium nickel manganese composite oxide having a spinel structure as a positive electrode active material of a lithium secondary battery. Among them, the problem is a serious problem to be solved particularly in the (5 V-class) manganese-based spinel-type lithium transition metal oxide having an operating potential of 4.5 V or more.
As the cause of such generation of gas, it has been believed that the gas is generated as the impurities contained in the positive electrode active material react with the electrolytic solution conventionally, and thus it is proposed a method to remove the water-soluble impurities by washing them with water.
For example, a method for producing a positive electrode active material for secondary battery in which a lithium compound, a manganese compound, and at least one kind of metal or metal compound selected from the group consisting of Ni, Al, Co, Fe, Mg, and Ca are mixed and calcined to obtain lithium manganese oxide, and the lithium manganese oxide is then washed with water, then filtered, and dried to obtain a positive electrode active material for secondary battery is proposed in Patent Document 5.
In addition, a method for removing impurities on the particle surface by washing the spinel-type lithium transition metal oxide obtained by calcination is proposed in Patent Documents 6, 7 and 8 as well.